Updated 7/17/2016 to account for Ephemera's name change to Ephemer.
Regarding how Yen Sid and Kairi don't want to consider the idea that Braig is an Uunchained, I thought it would be fun to introduce a bit of dramatic irony, and have the audience know that Braig is an Unchained, but the characters don't.
Chapter 24: Castle Oblivion:
Castle Oblivion – Outside the Castle
"So this is Castle Oblivion, huh?" Sora remarked as the five heroes stood before the foreboding green and brown castle that had once been Aqua's home and the Organization's secondary base. Even if he didn't remember being here, the feeling of familiarity he got when looking at the ominous structure only confirmed that he had. I wonder what we'll find in there, he thought to himself as Aqua led the way along the bleak stone pathway towards the citadel's front entrances. But whatever's in there, I can handle it. While he, Lea, and Kairi were being fitted for their new outfits, Riku, Mickey, Yen Sid, and Aqua discussed the layout of Castle Oblivion and what awaited them there. Mickey had originally believed that the illusions the Organization had used to manipulate Sora were Namine's doing, but Riku confirmed that this wasn't the case. Namine could manipulate the illusions because they were based on memories, but the illusions themselves were wholly the creation of Castle Oblivion itself.
Realizing that being near Ven's body would probably cause the illusions to mimic the boy's memories, Riku suggested that the trip to Castle Oblivion should be Sora's second chance to show the Mark of Mastery. Mickey had argued that it was too dangerous, but Riku had countered that the journey through the Realm of Sleep was equally dangerous. "Besides," he'd added. "As legitimate as Master Yen Sid's reasons for failing him are, he's still earned the title several times over after everything he's done." No one could argue with that statement, and so the other three Masters reluctantly agreed that braving the illusions of Castle Oblivion a second time would be the ideal opportunity for Sora to prove himself a Master, since without Namine to manipulate the illusions, Xehanort would not be able to manipulate events within the Castle the way he could in the Realm of Sleep.
When told that Castle Oblivion would be his second attempt at the Mark of Mastery, Sora had practically sprinted downstairs to the ship. After failing the last time, he was eager to prove that he had earned the rank of Master. Now, he stood at the threshold of Castle Oblivion for the second time in his life, preparing himself to face the challenges within and prove himself a Keyblade Master. With a nod to everyone else, he pushed open the doors leading to the entrance hall. Everyone strode into the hall, and Goofy shut the door behind them.
"So what exactly am I supposed to do for my exam?" Sora asked as he turned around to face the group.
"The Castle creates illusions based on the memories in your heart," Aqua explained. "Master Yen Sid just told me that you had to see what Ven had gone through. It will hurt, and the properties of the castle mean that as soon as you see Ven's memories, you'll forget them, and things won't exactly make sense. It'll be frustrating, and it'll make you angry. It may even hurt so badly that you feel like you're about to lose yourself. But don't worry, that won't happen to you. Mickey told me that you're like a sponge. No matter much much pain life throws at you, you'll suck it up and leave things a little bit better. Just follow your heart and you'll make it through. You don't have to do anything special. Do what you do – just be yourself. Riku tells me that's what you're good at, so just do what feels right for you."
"Alright," Sora said as he turned and reassured Kairi, Donald, and Goofy that he'd be alright before heading through the first door. After the doors closed, Aqua drew Master Eraqus' Keyblade and started waving it around the room until it glowed when she aimed it at a section of the wall just to the right of the door Sora had gone through.
"This way," she called out as she led the rest of the group toward the wall. As they got closer, a beam of light shot out from the tip of the blade, causing a section of the wall to open outward, revealing a secret passageway. "We'll meet Sora in the Chamber of Waking."
"Are you sure he'll be okay?" Kairi asked as she glance nervously at where Sora had left the room. "What if something goes wrong?" Donald and Goofy quickly nodded their agreement.
"If something goes wrong, then I'll go in and get him myself," Aqua answered as they made their way through the hidden passages of the Castle toward a hidden staircase that led directly to the top floor. "But I'm confident that he'll handle things with flying colors. I know you all want to go with him, but this is something he has to do by himself." As the group climbed the staircase, Kairi still couldn't help but feel worried about what Sora would see when he faced the illusions of Castle Oblivion.
Meanwhile, Sora had opened the doors only to find himself standing inside the Round Room in the World That Never Was. This must be the first illusion, he thought to himself as he stood on the raised central platform looking up at the tall white thrones of the Organization's meeting room. All thirteen seats were filled, including the one designated to Roxas, who just sat there with a blank look on his face.
"Good tidings friends," he heard Xemnas' voice call out from above. Sora turned to glare up at the Superior. "Today is a momentous day," the man continued, oblivious to the Keyblade wielder's presence. "I am pleased to announce that a new comrade has been chosen to wear the coat. Number XIV." Huh? Sora thought. Since when did the Organization have 14 members? He turned around to see a girl in a black coat, her face obscured by the hood of the uniform. Before the seventeen year old could say anything, the girl turned around to face Roxas, causing his Nobody to gasp. As quickly as it had begun, the scene before him ended, and Sora found himself walking out a pair of doors to find another set identical to the ones he'd just walked out of. But when he tried to reflect on what he'd witnessed, Sora found that he couldn't even remember what he had seen.
"What? Sora gasp aloud, confused. "Why can't I remember? I know everyone in there."
"Don't you remember?" A voice called out. Sora turned to find a figure wearing the Oragnization's signature black coat emerging from a corridor of Darkness. "Everything in this castle is just an illusion," the figure explained. "And once the illusions vanish, you'll forget all about the painful memories you've just seen.
"I'll forget?" Sora asked as he began to question the meaning of his test. How am I supposed to pass if I can't even remember what I'm seeing? He asked himself.
"Hey, no big deal," the figure in black replied. "The fact that you can't remember just means none of what you saw was very important to you to begin with. Isn't that right?"
"That's not true!" Sora snapped, angry that this person would even suggest such a thing. "Of course it's important to me. Okay maybe I forgot what just happened and who I saw inside that room. But trust me, I know I'll remember everything again."
"Sure," the figure retorted sarcastically. "By the way, you can go in and fight everyone if you want to?"
"What!?" Sora exclaimed, getting even more confused.
"What do you care?" the mysterious figure retorted bitterly. They're only strangers. Actually, they're just illusions of strangers. They'll only be there till they're not. Do you see where I'm going with this? Nothing's real. You can break their little non-existent hearts and forget about it. No hurt feelings, no baggage, it's that simple. You're 100 percent guilt free."
"That's what you think" Sora replied angrily.
"What?" Now the figure sounded confused.
"It's obvious you know nothing at all," the teenage Keyblade Wielder continued. "Everything you say is a lie. I might have the strength to hurt someone, but what exactly would be the point? I know that it would tear me up inside. That ache would stay with me for a long time. You might not know it now, but I hope you'll see that nothing's guilt-free in this world."
"You've arrived at the first question," the figure said ominously before silently fading into thin air, leaving only a confused Keyblade Wielder in their wake.
The World That Never Was: Grey Area
"They're at Castle Oblivion," Isa reported from where he stood near the large floor-to-ceiling windows of what used to be the old Organization's break room. Master Xehanort nodded solemnly.
"So they plan on attempting to reawaken Ventus," the old man replied calmly before turning to his one-eyed henchman. "Braig, follow them and see to it that you get to Ventus first. A body without a heart will be the perfect vessel to complete the circle."
"With pleasure," the one-eyed man replied before disappearing into a dark corridor. He emerged inside the sterile white halls of Castle Oblivion.
Perfect, Braig thought to himself as he opened the first door. Now all I have to do is catch up to them before the illusions kick in.
"Ephemer?" A girl's voice called out. Me and my big mental mouth, He chastised himself as he turned to look upon the illusion of the person who had called out his original name. Despite knowing it wasn't real, he couldn't help but gasp in shock.
"Ava?"
The Ephemer bit will only make sense if you've either played Kingdom Hearts X, read story summaries, watched English-translated playthroughs on YouTube, or is up to date on Unchained X.
The bit about Namine not creating the illusions of Castle Oblivion and only influencing them is my head canon on the matter because yes, Namine can manipulate Sora's memories, she's never been shown to actually create illusions. So I surmised that the illusions themselves were the creation of Castle Oblivion, while Namine could only influence them because of her power over Sora's memories.
I was originally just going to have them walk right through and have them rescue Ventus all in one chapter. But then I started thinking about when to have Sora retake his Mark of Mastery Exam. But then I started thinking about the reason that Sora failed the exam despite everything that he's done while Riku passed, and I realized that the reason he failed was because despite all his power and accomplishments, he was so used to dealing with light that he had no experience protecting his heart against darkness. So I decided that having him face the memories of all the people inside him in what is essentially a mirror of the Castle Oblivion section of Re:coded would be the best chance to prove himself. (I actually borrowed the dialogue because I felt it would be interesting to have the real Sora's journey mirror Data-Sora's)